That's interesting. I thought the zerg campaign would be more about trying to find a new zerg leader and trying to rebuild after the defeat on Char. However, I guess I could see Kerrigan having to return to the zerg in order to somehow defeat the dark shade (thinking about the prophecy). There are a lot of options.

1.) Xel'naga create overmind > overmind's purpose in life is to control zerg to kill protoss.

2.) Terran and Protoss in the first starcraft games go their best to wipe out the zerg and kill the overmind, but it is revealed to be a somewhat false-victory in Starcraft II because:

3.) The overmind is revealed to be a "Chaotic Good" type of character in the zeratul missions in Starcraft II, where he makes Kerrigan "the queen of blades" as an act of defense against "The Fallen One" // Xel'naga // hybrids.

4.) Overmind > through Tassadar tells Zeratul to tell Jim raynor of Kerrigan's importance and that she must not be killed, and thus she is saved at the end of Wing's of Liberty.

5.) Tycus was sent to assassinate Kerrigan. Who was he working for? The Hybrid in the secret mission is found within a Dominion laboratory full of protoss and zerg prisoners they were experimenting on - Who is Mensk working for?

6.) the protoss mission where everybody makes a last stand and dies is the future in which Kerrigan is dead, so her being saved at the end of Wing's of Libery prevents that timeline, and pushes fourth a new one.

So essentially there is a greater conflict that ends up being Protoss + Zerg + Terran vs. Xel'naga. The three races sure do seem to kill eachother way too much if this is the case in my opinion, but we'll see how the story gos. As I understand it the zerg expansion is to detail the events at the same time as Wing's of Liberty, except from Kerrigan's side.

She is going to become the queen of blades again, but this time her mind is intact and she will still be "normal" Kerrigan.

This is what it said on the Starcraft 2 wiki for Heart of the Swarm campaign features.

Clue #1"The campaign is focused on Kerrigan and the development of a zerg empire. The campaign operates similarly to an RPG, in which Kerrigan is focused on increasing her personal power through mutations."

As you can see here, Kerrigan will be mutating back to the Queen of Blades to increase her power, aka leveling up.

Clue #2"Zerg armies protect her as she garners this power. The greater her power, the more zerg she's able to control."

As you can see here, she will be having to mutate back into the Queen of Blades to control more of the zerg units. Something like Wings of Liberty's unit unlock system.

Clue #3"Like Wings of Liberty, the player will be presented with choices as to which missions to carry out and which units to get, but such choices won't be based on cash."

Similar to WoL and you can choose etc...

Extra Info:"Both the missions and their objectives are intended to be quite different from the ones featured in Wings of Liberty."

All backed up by blizzard sources.

starcraft.wikia.com/wiki/Heart_of_the_Swarm

So as you can see, Kerrigan will have to turn back into "infested" Kerrigan to gain full control of all the zerg units again.

So I'm guessing with her hair still in place, the first few units she can control are drones, larvas and most likely hydralisk/zerglings. Then you can unlock more of the other units as you progress in the story.

Even though she's no longer Zerg, Kerrigan is still the most powerful psionic force in the sector.

The Confederacy proved that the Zerg can and will respond to a human psionic giving orders, so I'm guessing that Kerrigan will retain most of her control over the Swarm, just that she'll be human henceforth.